| 1. |
- Buixaderas, E., et al.
(författare)
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Broadband dielectric response and grain-size effect in K0.5Na0.5NbO3 ceramics
- 2010
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 107:1, s. 14111-
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Tidskriftsartikel (refereegranskat)abstract
- Dielectric spectra of two K0.5Na0.5NbO3 ceramics with different grain sizes (10 and 0.5 mu m) were measured from 10(2) to 10(14) Hz in a broad temperature range. The sequence of first-order phase transitions (cubic-tetragonal-orthorhombic-rhombohedral) was detected by differential scanning calorimetry, dielectric spectroscopy, and time-domain terahertz spectroscopy. The grain size affects all the phase transitions, which are more smeared in the small-grain sample. In the large-grain ceramics, two well-separated near-Debye relaxations are seen in the tetragonal phase, which suddenly merge on cooling across the tetragonal-orthorhombic transition, and on further cooling the lower-frequency relaxation strongly broadens. On reducing the grain size, the higher-frequency relaxation shifts from similar to 1 to similar to 20 GHz and the lower-frequency one strongly broadens. Without quantitative understanding, these effects could be assigned to domain-wall dynamics and its temperature and grain-size dependences. Similar to pure KNbO3, an overlapped central-mode-soft-mode type excitation was detected in the terahertz range related to the effective hopping and oscillations of the off-centered Nb ions in a multiwell potential.
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| 2. |
- Fekete, L., et al.
(författare)
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Fast one-dimensional photonic crystal modulators for the terahertz range
- 2007
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Ingår i: Optics Express. - 1094-4087. ; 15:14, s. 8898-8912
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Tidskriftsartikel (refereegranskat)abstract
- Optically controlled one-dimensional photonic crystal structures for the THz range are studied both theoretically and experimentally. A GaAs:Cr layer constitutes a defect in the photonic crystals studied; its photoexcitation by 800 nm optical femtosecond pulses leads to the modulation of the THz beam. Since the THz field can be localized in the photoexcited layer of the photonic crystal, the interaction between photocarriers and THz light is strengthened and yields an appreciable modulation of the THz output beam even for low optical pump fluences. Optimum resonant structures are found, constructed and experimentally studied. The dynamical response of these elements is shown to be controlled by the lifetime of THz photons in the resonator and by the free carrier lifetime. The time response of the structures studied is shorter than 330 ps. (c) 2007 Optical Society of America.
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| 3. |
- Fekete, L, et al.
(författare)
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Ultrafast opto-terahertz photonic crystal modulator
- 2007
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Ingår i: Optics Letters. - 0146-9592. ; 32:6, s. 680-682
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Tidskriftsartikel (refereegranskat)abstract
- We present an agile optically controlled switch or modulator of terahertz (THz) radiation. The element is based on a one-dimensional photonic crystal with a GaAs wafer inserted in the middle as a defect layer. The THz electric field is enhanced in the photonic structure at the surfaces of the Ga-As wafer. Excitation of the front GaAs surface by ultrashort 8 10 nm laser pulses then leads to an efficient modulation of the THz beam even at low photocarrier concentrations (approximate to 10(16) cm(-3)). The response time of the element to pulsed photoexcitation is about 130 ps. (c) 2007 Optical Society of America.
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| 4. |
- Kuzel, P., et al.
(författare)
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Propagation of terahertz pulses in photoexcited media: Analytical theory for layered systems
- 2007
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 127:2
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Tidskriftsartikel (refereegranskat)abstract
- Optical pump-terahertz probe spectroscopy has become a widely used experimental tool for the investigation of the ultrafast far-infrared response of polar systems. In this paper the authors present an analytical method of calculating the propagation of ultrashort terahertz pulses in photoexcited media. The transient terahertz wave form transmitted through the sample is equal to a product of the incident terahertz field (at a mixed frequency), transient susceptibility, and a so called transfer function which depends on the properties of the sample in equilibrium. The form of the transfer function is derived for general layered systems and for specific cases including one-dimensional photonic crystals, thin films, and bulk samples. Simplified expressions directly applicable to the analysis of the experimental results related to the most common sample geometries are shown and discussed. (C) 2007 American Institute of Physics.
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